1. Field of the Invention
This invention relates generally to microlens arrays and image sensors.
2. Description of Related Art
Light sensors are often combined in either linear or two-dimensional configurations for the purposes of image detection. Individual light sensors within the image sensor provide output signals whose magnitudes correspond to the light intensity incident upon them from a given area of the source object. The detected light intensity and corresponding output signal pattern of the light sensors thus forms a linear or two-dimensional representation of the source object or image.
In many situations, solid-state image sensors such as charged coupled devices (CCD's), complementary metal-oxide-semiconductor (CMOS), and other solid-state electronic devices are required to detect low or marginal light intensities. In these instances, even small losses in detectable light are of significant importance. Various means are available to the image sensor system designer for enhancing the system's ability to detect low light signals. These means often include (1) the increase of the light detection area of the sensor, (2) the addition of active circuitry for signal amplification, (3) the time integration of the light signal, (4) the increase of light collected by the imaging optics, and (5) the addition of microlenses to collect more of the incident light. All of these techniques are valuable and commonly employed, dependent upon the application and technology used for detection.
Typically an individual light sensor comprises a light-detection area and a non-light-detection area with associated supporting items such as electronic circuitry, interconnecting metal traces, and isolation diffusions. The light sensor often occupies a square or rectangular area of the image sensor. This area is referred to as a pixel. Light incident upon the non-light-detection area within the pixel is lost unless some means, such as a microlens, is employed to deflect it to the light-detection area of the light sensor, thereby collecting light that would otherwise have been lost. Microlenses typically have circular cross-sections in a plane parallel to the detection plane of the light sensor. Other cross-sections, however, have been proposed. The perimeter of the microlens is inscribed within the boundaries of its associated pixel and obviously collects light only within that perimeter. Previous systems have not attempted to collect light from the corner areas of the pixel.
As an example of the light which can potentially be lost from a pixel, refer to FIGS. 1A-1D and TABLE A. From FIG. 1A and TABLE A, it is seen that a circular lens inscribed within a square pixel and occupying the maximum possible area will be unable to collect approximately 21.5% of the incident light due to the inability of the circular cross-section of the microlens to cover the corners of the pixel. Referring to TABLE A, the square collection area of FIG. 1B, the hexagon collection area of FIG. 1C, and the octagon collection area of FIG. 1D respectively lose approximately 50%, 35.1%, and 17.2% of the light incident upon their corner areas. The particular geometries of FIGS. 1A-1D were chosen for illustrative purposes.
From the above discussion, it is clear that there is a need for further improvements in the percentages of available light collected in image sensors and similar light-detection devices.
TABLE A ______________________________________ Fractional Loss of Corner Area Incident Light Lens Cross-Section: Circle Square Hexagon Octagon ______________________________________ Fractional Loss: 0.215 0.500 0.351 0.172 ______________________________________ As = Area of Square Pixel Anc = Maximum NonCorner Area of Light Collection Fractional Loss of Corner Area Incident Light = 1 - (Anc/As)